In my youth I got a job in a gold mine. I welded all sorts of scrap metal together to make chutes, bins, conveyor arms, etc. I used whatever rod I could find. The finished job looked a treat. The owner showed up and the foreman and I dumped the first load of rock in the hopper and let it run down through the line. A bunch of my beautiful butt welds snapped and everything went to pieces. A check was written, handed to me, and I was told to go. The only job I have been fired from.
Butt welds are successfully used in the manufacturing world. They do structural designs to make sure the welds are not overloaded. They know the composition and temper of the steel they are using and the makeup of the rod. Samples are welded and tested in the lab. Certified welders do the work. There is quality control such as x-rays. I did not do this at the gold mine and most home builders do not either.
Willie is right. Inserts and gusset plates are extra work but they can save a person's hide, especially with our seat of the pants design and engineering.